The present invention relates generally to semiconductor devices, and more particularly, to a semiconductor device structure with active regions having different surface directions and local mechanical stress, and a method of forming the same.
Performance improvement of semiconductor devices is a never-ending endeavor for manufacturers of those devices. One of many mechanisms to improve performance of semiconductor devices is to apply local mechanical stress to particular devices. That is, stress that is mechanically applied to localized areas of a semiconductor device. One challenge in providing these stresses, however, is that the optimal stress type and direction relative to current flow direction is different for the two types of devices commonly used on semiconductors, i.e., n-type field effect transistors (nFETs) and p-type field effect transistors (pFETs).
In terms of current flow, conventional complementary metal-oxide semiconductor (CMOS) technologies use wafers with (100) surface orientation, and a notch in the <110> direction so that the current flow direction for nFETs and pFETs is in the <110> direction. In terms of applying mechanical stress to devices, piezoresistive coefficients have been measured for silicon (Si) with a (100) surface orientation. These measurements indicate that an optimal nFET has a current flow in the <100> direction with a tensile stress in a longitudinal direction and a compressive stress in the transverse direction relative to the current flow direction. In contrast, an optimal pFET has been found to have current flow in the <110> direction with a compressive stress in the longitudinal direction and a tensile stress in the transverse direction relative to the current flow direction. Based on these facts, applying the optimal stress type and direction relative to current flow direction is unfeasible using conventional technology.
Another challenge to achieving these optimal structures is presented by the state-of-the-art lithography requirement that gate electrodes must be oriented in a single orientation, e.g., for static random access memory (SRAM). Conventional technology does not allow achievement of both the best stress type and current flow directions, and single gate electrode orientation.
In view of the foregoing, there is a need in the art for a structure and method to simultaneously achieve the optimal stress type and direction with respect to current flow for both types of devices, and for gates orientated in one direction.